A generally used positive working photoresist composition comprises an alkali-soluble resin and a naphthoquinonediazide compound as a photosensitive material. For instance, the combinations of novolak-type phenol resins with naphthoquinonediazido-substituted compounds are disclosed, e.g., in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470, and the combinations of novolak resins prepared from cresols and formaldehyde with trihydroxybenzophenone-1,2-naphthoquinonediazide sulfonic acid esters are described as most typical compositions in L. F. Thompson, Introduction to Microlithography, No. 219, pp. 112-121, ACS publishing.
Novolak resins as binder are particularly useful for the present purposes, because they are soluble in aqueous alkali solutions without swelling therein and can provide high resistance to etching, especially to plasma etching, when the images formed therefrom are used as etching mask. In addition, naphthoquinonediazide compounds used as photosensitive material are unique in having a property that the compounds themselves function as dissolution inhibitor to lower the solubility of a novolak resin in alkali, but, when irradiated with light, they decompose to produce alkali-soluble substances, and thereby to rather promote dissolution of a novolak resin in alkali. Owing to such a great change produced in property by light, naphthoquinonediazide compounds are particularly useful as the photosensitive material of positive working photoresist compositions.
From these points of view, a large number of positive working photoresist compositions comprising a novolak resin and a photosensitive material of naphthoquinonediazide type have hitherto been developed and put to practical use. In the processing of fine patterns having a line width of the order of 1 .mu.m at the thinnest, those compositions have accomplished satisfactory results.
With respect to integrated circuits, on the other hand, the integration degree thereof has become higher and higher in recent years. Thus, processing of superfine patterns having a line width of no broader than 1 .mu.m, particularly no broader than 0.5 .mu.m, has come to be required in the manufacture of semiconductor substrates for VLSI and the like. In order to answer such a purpose, it is required for photoresist to have very high resolution, wide defocus latitude, highly precise reproducibility of pattern shape by which exact copying of the shape of an exposure mask is enabled, and wide development latitude enough to ensure a consistent line width upon processing. In order to achieve high productivity, it is further required for the photoresist to have high sensitivity. In addition, the resist is required to have high heat resistance. This is because dry etching processes are supplanting conventional wet etching processes for the purpose of heightening the integration degree of integrated circuits, and dry etching involves a rise of the resist temperature to cause thermal deformation of the resist.
Hitherto, the use of resist having a high .gamma. value has been considered to be advantageous for heightening resolution and reproducing an image of desirable pattern shape, and so the art of resist compositions capable of answering such purposes have been developed. Thus, there are known a large number of patents and reports disclosing such an art. In particular, many applications have been made for patents with respect to the art of novolak resins as a main component of positive working photoresist, including their monomer compositions, molecular weight distributions and syntheses methods, and those patents have accomplished certain results. However, the drawback of being low in sensitivity is common to most of such high resolution resists. As the art of heightening the sensitivity, on the other hand, the addition of low molecular weight compounds having alkali-soluble groups as dissolution accelerator is disclosed, but on the contrary the addition of such compounds often brings about the problems of narrowing the development latitude, lowering the heat resistance and so on. Further, many of those compounds have disadvantages of impairing the pattern shape and so on.
Thus, it is desired to discover resists having not only wide development latitude but also high sensitivity and satisfactory resolution, defocus latitude and heat resistance. The term "development latitude" as used herein can be defined as the dependence of a line width of the developed resist upon the development time or the development temperature. The term "defocus latitude" as used herein can be expressed in terms of the change caused in a resist line width by a shift of the position on which the focus is taken, or in terms of the focus range at which a resist pattern isolates without decreasing film thickness.